Nitrogen fixation in root nodules of Alnus is brought about by

Options

A. Frankia
B. Azorhizobium
C. Bradyrhizobium
D. Clostridium

Explanation

### Step-by-Step Solution:

1. **Understanding Nitrogen Fixation**: Nitrogen fixation is the process by which nitrogen (N2) in the atmosphere is converted into ammonia (NH3) or related nitrogenous compounds in soil and water. This is crucial for plants as they cannot utilize atmospheric nitrogen directly.

2. **Identifying the Context**: The question specifically refers to nitrogen fixation in the root nodules of the genus Alnus, which is a type of non-leguminous plant. It is important to note that not only legumes but also some non-leguminous plants can form root nodules for nitrogen fixation.

3. **Examining the Options**:
- **Option A: Francia**: This is a genus of actinomycetes that is known to form symbiotic relationships with Alnus species, facilitating nitrogen fixation in their root nodules.
- **Option B: Azorhizobium**: This bacterium is known for forming nodules in the aquatic plant Cajanus cajan (pigeon pea) and does not associate with Alnus.
- **Option C: Bradyrhizobium**: This bacterium forms symbiotic relationships primarily with legumes such as soybean and does not play a role in Alnus.
- **Option D: Clostridium**: This is a genus of anaerobic bacteria that can fix nitrogen but does not form nodules in Alnus.

4. **Conclusion**: Based on the information about the symbiotic relationship between Alnus and nitrogen-fixing bacteria, the correct answer is **Option A: Francia**.

### Final Answer:
**Francia**

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